A surface texture measuring instrument for measuring surface texture of a work and a contour measuring instrument for measuring a surface profile (contour) of a work are widely used. The surface texture measuring instrument detects minute recesses and protrusions on the work surface and detects a change in height along a minute length on the work surface, i.e., a change in height in a short period. In contrast, the contour measuring instrument detects a change in height in a comparatively long period on the work surface. In other words, the difference between the surface texture measuring instrument and the contour measuring instrument lies in the minuteness of displacement of the stylus to be detected, i.e., the period range to be detected of the period component corresponding to the length in the detection signal obtained when relatively moving the detection unit at a fixed speed with respect to the work. Because of this, the displacement detector (sensor) used in the surface texture measuring instrument is required to have high-speed responsibility and to be capable of detecting a minute displacement, i.e., to have a high resolution, however, is not necessary to have such strict detection accuracy with regard to the absolute value of the displacement in a long period, i.e., the linearity of the detection signal in a wide detection range. In other words, the linearity of the detection signal in a wide detection range is allowed to have comparatively low accuracy. In contrast to this, the displacement detector (sensor) used in the contour measuring instrument is not required to have so high-speed responsibility as in the case of the surface texture measuring instrument and is not necessary to be capable of detecting a minute displacement (high resolution), however, is necessary to have high accuracy with regard to the absolute value of displacement in a long period, i.e., the linearity in a wide detection range of the detection signal.
In general, by taking into consideration the characteristics of the measurement as described above, the moving speed when relatively moving the detection unit with respect to the work is different between the case where surface texture is measured and the case where the contour is measured. Specifically, the moving speed in the case where surface texture is measured is low compared to the moving speed in the case where the contour is measured.
The contour measuring instrument and the surface texture measuring instrument have similar configurations and a measuring instrument capable of measuring both the surface profile (contour) and the surface texture is demanded.
FIG. 1 is an external appearance diagram of a contour and surface texture measuring instrument.
As illustrated in FIG. 1, a contour and surface texture measuring instrument 1 has a base 2, a strut 3 provided on the base 2, an X-axis drive unit 4 slidably supported by the strut 3 in a Z-axis direction, an X arm 5 supported by the X-axis drive unit 4 movably in an X-axis direction, a measurement part 6 provided at the tip end of the X arm 5, and a mounting table 8 provided on the base 2.
In the case where measurement is performed, a stylus 7 provided at the tip end part of the displacement detector 6 is caused to come into contact with the surface of a work W mounted on the mounting table 8 with a fixed force. In this state, if the arm support unit 5 and the displacement detector 6 are moved along the X axis by the X-axis drive unit 4, the stylus 7 changes its position in the Z-axis direction in accordance with the profile of the surface of the work W. The displacement detector 6 outputs an electric signal in accordance with the displacement of the stylus 7 by a built-in sensor, for example, such as a differential transformer.
FIG. 2 is a diagram illustrating a configuration example of the displacement detector 6 in the case where a differential transformer is used and an example of a detection signal: in FIG. 2, (A) illustrates a configuration example and (B) illustrates an example of a detection signal.
As illustrated in (A) of FIG. 2, the displacement detector 6 has a holder 14 rotatably supported by a pivot 16 engaged with a case, an arm 12 locked to the holder 14 in an attachable and detachable manner, the stylus 7 provided at the tip end of the arm 12, and a differential transformer-type detection mechanism (sensor) 15 configured to output a signal in accordance with the displacement of the holder 14. A portion attached to the tip end of the arm 12 and including the stylus 7 is called a probe 13. The differential transformer-type detection mechanism (sensor) 15 has a fixed portion including a plurality of coils fixed to the displacement detector 6 and an iron core portion attached to the holder 14 and the position of the iron core portion relative to the plurality of coils of the fixed portion changes due to the rotation of the holder 14 and the intensity of an alternating-current signal (detection signal) that occurs in the coil changes. The differential transformer-type sensor is widely known, and therefore, more explanation is omitted.
When the probe 13 is attached to the holder 14 and the stylus 7 is caused to come into contact with the work surface under a predetermined pressure, in accordance with the contact position, i.e., in accordance with the height and the recesses and protrusions of the work surface, the probe 13 and the holder 16 rotate, the iron core portion of the differential transformer-type sensor changes its position, and a detection signal in accordance with the displacement is output. The intensity of the detection signal of the differential transformer-type sensor changes substantially in proportion to the displacement and changes in accordance with a very minute displacement, but the intensity does not change perfectly in proportion to the displacement and as illustrated in (B) of FIG. 2 and a difference from the value in the case of the perfect proportion increases on both sides of the detectable range, i.e., the linearity deteriorates.
Because of this, in the surface texture measuring instrument required to be capable of detecting a minute displacement, i.e., to have a high resolution, but not required to have so strict detection accuracy with regard to the absolute value of a displacement in a long period, the differential transformer-type sensor is used in many cases.
In the case where the differential transformer-type sensor is used in the contour measuring instrument, calibration of the differential transformer-type sensor is performed in advance and by preparing a correction table storing a deviation between displacement and detection signal to make correction, the linearity is improved. However, the differential transformer-type sensor is susceptible to the change in temperature and it is difficult to implement sufficient linearity only by this correction.
On the other hand, as a displacement detector having high linearity in a wide detection range, a scale-type detection mechanism (sensor) is known. The scale-type sensor has a scale recording graduations and detects the amount of change in graduation accompanying movement or the position to which the scale has moved. The scales are of several kinds: an optical scale, a magnetic scale, etc.
FIG. 3 is a diagram explaining an optical scale-type detection mechanism (sensor): in FIG. 3, (A) illustrates an example of graduations of the scale used in the optical scale-type sensor, (B) illustrates a configuration example of the detection unit of the optical scale-type sensor, and (C) illustrates an example of the detection signal, respectively.
As illustrated in (A) of FIG. 3, the graduations of a scale 21 form a black and white pattern formed on a glass plate, etc., and the black portion is formed by vapor deposition of chromium etc.
As illustrated in (B) of FIG. 3, a detection unit is provided so as to sandwich the scale 21 that moves. In the detection unit, a light source 22, such as an LED and laser, and a lens 23 that converges light from the light source 22 onto the surface on which the black and white portions of the scale 21 are formed are provided on one side of the scale 21, and a lens 24 that collects light having passed through the scale 21 and a light receiving element 25 that detects light collected by the lens 24 are provided on the other side of the scale 21.
The amount of light received by the light receiving element 25 changes depending on whether the white portion or the black portion of the scale 21 is located on the portion of the light flux collected by the lens 23 when the scale 21 moves. Therefore, a detection signal that changes as illustrated in (C) of FIG. 3 is obtained. By processing this detection signal, it is possible to detect the amount of movement of the scale 21 or the position to which the scale 21 has moved.
FIG. 4 is a diagram illustrating a configuration example of a displacement detector that uses the optical scale-type sensor. As illustrated in FIG. 4, the displacement detector has a parallel link mechanism in which link members 34 and 35 are linked by two links 36 and 37. The links 36 and 37 are engaged with four rotation shafts of the link members 34 and 35 and it is possible for the link members 34 and 35 and the links 36 and 37 to have a state where the link members 34 and 35 are kept parallel and the links 36 and 37 are kept parallel, i.e., to deform so as to form a parallelogram. One rotation shaft 38 of the four rotation shafts is engaged with a case of the displacement detector and the link 36 is rotatably supported with the rotation shaft 38 as a pivot. An arm 32 at the tip end of which the stylus 7 is provided is locked by the link 36. Consequently, the arm 32 and the link 36 are rotatably supported by the rotation shaft (pivot) 38 in the same manner as the arm 12 and the holder 14 in FIG. 3.
On one side of the link member 35, an optical scale-type sensor 39 is provided, which detects a displacement of the link member 35. In FIG. 4, the scale having graduations is fixed to the case and an index scale, to be described later, is provided to the link member 35. It is also possible to provide a scale to the link member 35 and to provide the detection unit to the case.
In the displacement detector in FIG. 4, the link member 35 moves a small amount in the transverse direction, however, moves parallel, and therefore, it is possible to use the optical scale-type sensor using the scale having graduations as illustrated in (A) of FIG. 3. However, the parallel link mechanism illustrated in FIG. 4 requires a large space.
Because of this, as illustrated in (A) of FIG. 5, in the displacement detector in FIG. 2, a scale 17 in which patterns are formed radially is provided in the holder 14 and by detecting the amount of rotation (rotation position) of the holder 14 by utilizing the scale 17, the displacement of the stylus 7 is detected. In the scale 17, as illustrated in (B) of FIG. 5, black and white patterns with the pivot 16 as the center are formed into an arc shape. In the case where the amount of movement of the pattern in the arc shape is detected also, it is possible to use the same method as that explained in FIG. 3. Further, a method has also been proposed, in which the surface on the rear end side of the holder 14 is formed into a cylindrical surface with the pivot 16 as the center and patterns at regular intervals are formed thereon, and thereby the amount of movement (rotation) of the pattern is optically detected.
As the resolution of the scale-type sensor, basically, the resolution is specified by the pitch of the graduations and various kinds of methods for improving the resolution by using an index scale, etc., have been proposed. Further, the method in which the graduations of the scale 21 are formed as a diffraction grating and the resolution is improved by laser interference has also been proposed. However, the method for forming graduations into a diffraction grating and improving the resolution by laser interference requires a large-scaled configuration and the size thereof becomes large, and therefore, it is difficult to use the method in the displacement detector of the contour/surface texture measuring instrument. There is also such a problem that the configuration to implement this method is complicated, and therefore, expensive.
In any method, the scale-type sensor uses the scale as a reference, and therefore, displacement detection with high accuracy is possible in a wide range, but it is difficult to obtain a resolution as high as the resolution of the differential transformer-type sensor.
A laser interferometer system has been known as a displacement detector capable of displacement detection with high accuracy in a wide range and having a high resolution.
FIG. 6 is a diagram illustrating a configuration of a displacement detector that uses the laser interferometer system. As illustrated in FIG. 6, in the displacement detector in FIG. 2, the holder 14 is provided with a corner cube 43 forming a laser interferometer but not provided with the differential transformer-type detection mechanism (sensor). The laser interferometer has a light source (laser) 41, a beam splitter 42, the corner cube 43, a corner cube 44, two reflection mirrors 45 and 46 provided to the beam splitter 42, and a light receiving element 47.
The laser beam emitted from the light source 41 is split into two beams by the beam splitter 42. One of the split laser beams is reflected from the corner cube 43 and further reflected from the reflection mirror 42 and returns to the corner cube 43, and then is further reflected and enters the beam splitter 42, and is reflected and travels toward the light receiving element 47. The other split laser beam is reflected from the corner cube 44 and further reflected from the reflection mirror 46 and returns to the corner cube 44, and then is further reflected and enters and passes through the beam splitter 42 and travels toward the light receiving element 47. The two laser beams traveling from the beam splitter 42 toward the light receiving element 47 interfere with each other. When the holder 14 changes its position and the corner cube 43 changes its position, the light path length of one of the laser beams changes by four times the amount of displacement of the corner cube 43, and due to this, the difference between the light path lengths of the two laser beams incident on the light receiving element 47 changes and the state of interference changes. One change in light and darkness of interference in the light receiving element 47 corresponds to one wavelength of the laser beam, and therefore, by detecting the one change of the detection signal of the light receiving element 47, it is possible to detect the displacement of the corner cube 43, which is ¼ of one wavelength of the laser beam. If one wavelength of the laser beam is taken to be about 800 nm, it is possible to detect a displacement of 200 nm, and this is a very high resolution. Further, the detection range is also very wide and the linearity is good.
As described above, the displacement detector using the laser interferometer has a high resolution and good linearity, but is very expensive and the assembly and adjustment thereof are complicated.